So a few days ago, I had a dream. This dream was about an old dog I had and a girl I am infatuated with (that sounds extremely stalkerish but I mean it as I'm really in love with her) We'll call the girl "S" and I can only vividly remember the dream but one part was I was walking my dog and it was about 4 minutes later and S was there being hassled by some guy.
I ended up stabbing the guy repetitively and that's where it ended.
So to anyone good at psychology... Is this me going insane or wanting to hurt people. Or is it just me wanting to be able to protect S. Or because I have had many events of De Ja Vu is it me seeing into the future. 

Or maybe you know of someone getting close to her, and that you view that someone as a potential rival in love so you don't like him and wishing to get rid of him... there's nothing really sure when it comes to dream interpretation.

According to my notes, your dream could be some sort of wish fulfillment or your brain putting objects and sequence of your dreams in order.

(I'd like to point out that my notes only contain a very general idea of what dreams are; therefore, I can't go any further than what I'm about to and have written.)

Since you're deeply infatuated with S, perhaps deep down, you do want to protect her. Even if it means hurting others. It could be a desire you have, and your dream is showing an example of it.

Or maybe it was just coincidence. Maybe you dreamed of S, an old dog of yours, the guy that is bothering S, and a knife. How do you suppose your brain will put all of those random things in order? Into a story, of course! You dreamed that you walked your dog, saw S being harassed by a guy, and because you already have a conscience of being infatuated with her, you did what was instinctive (in the dream) and stabbed the guy, since you apparently had a knife (I assume) to stab him with.

And of course, there is the possibility of it being both wish fulfillment and order, as well.

As for seeing into the future with dreams? I don't know about that. What evidence is there for that, if any? I'd like to think that things like that are coincidental.